A new class of mid-wave infrared (MWIR) detector, termed an nBn detector, has recently been developed. See “nBn detector, an infrared detector with reduced dark current and higher operating temperature,” S. Maimon and G. W. Wicks, Applied Physics Letters 89, 151109 (2006), hereinafter referred to as “Maimon”. As described in Maimon, an nBn detector comprises an MWIR absorption n-type semiconductor, a large bandgap undoped barrier layer, and a second thin n-type layer. The barrier bandgap is larger than that of the absorption or contact layers. The thickness of the absorption n-type layer is about an optical absorption length or two. The barrier layer is thick enough so that there is negligible electronic tunneling through it, and the potential height of the barrier layer is such that there is negligible thermal excitation of majority carriers over it. The second n-type layer serves as a contact layer. In operation, metal contacts are applied to the n-type layers and a potential difference is applied to these metal contacts.
FIG. 1 illustrates a band diagram for the type of nBn detector described in Maimon. That part of the band diagram associated with the barrier is identified as such and is labeled with the letter “B”, and that part of the band diagram associated with the two n-type layers are labeled with the letter “n”, where the absorption layer is labeled as such and the other thin contact layer is labeled as such.
A potential difference is applied to metal contacts 102 and 104, where metal contact 102 is held at a positive potential with respect to that of metal contact 104. Illustrated in FIG. 1 are conduction band 106 and valence band 108. Pictorially illustrated in FIG. 1 is the generation of a hole-electron pair comprising hole 110 and electron 112 due to the absorption of a photon. Because of the potential difference between metal contact 102 and metal contact 114, electron 112 moves toward metal contact 102, and hole 110 moves toward metal contact 104. The current so generated by the absorption of infrared photons is sensed by an electronic circuit (not shown) coupled to the detector of FIG. 1.
The heterojunctions between the barrier layer and the two n-type layers are such that all of the bandgap difference appears in the conduction band offsets. That is, there is essentially zero offset in the valence band. This allows the barrier to block the flow of majority carrier current, while allowing the flow of minority carriers. As further described in Maimon, this type of structure significantly reduces dark current, e.g., Shockley-Reed-Hall (SRH) current and surface currents. When compared to many other types of photo diodes, this structure results in less noise, so that the nBn device may operate at a higher temperature with the same performance, or may provide better performance at the same temperature.
The requirement in Maimon of valence band alignment in the heterojunctions imposes a constraint on the type of alloy compositions used in the n-type layers and in the barrier layer. This requirement limits the device to only certain infrared wavelengths, e.g., 3.4 and 4.4μ cutoff wavelengths. Utilizing compositions that lead to a potential barrier on the valence band impedes the transport process of the minority carriers (holes) through the barrier layer. The resulting hole traps impede the minority carrier transport, which degrades the performance of the detector with regard to dark current reduction and operating temperature. This is illustrated in FIG. 2, where the upper part of the valence band in the barrier layer labeled 202 is for the case of Type II band alignment, and the lower part of the valence band labeled 204 is for the case of Type I band alignment. The potential barrier at the valence band impedes the minority hole transport. Furthermore, band-to-band tunneling may also take place.